Method of magnetically processing an iron-carbon alloy

ABSTRACT

A magnetic field assisted processing method entails heating an iron-carbon alloy at an austenitizing temperature for a time duration sufficient for the alloy to achieve an austenitic microstructure; cooling the iron-carbon alloy to an intermediate temperature defined by a continuous cooling transformation (CCT) diagram for the iron-carbon alloy at a rate sufficient to avoid phase transformation of the austenitic microstructure, the intermediate temperature being below a bainitic knee of the CCT diagram and above a martensite start temperature; and applying a high field strength magnetic field of at least about 0.2 Tesla to the iron-carbon alloy after reaching the intermediate temperature. The field is applied for a time duration sufficient to transform the austenitic microstructure into a fine dispersion of one or more iron carbide phases in a ferrite matrix in order to produce a magnetically-processed alloy having improved ductility and strength.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described in this disclosure was made with government support under Prime Contract Number DE-AC05-000R22725 awarded by the Department of Energy. The government has certain rights in this invention.

TECHNICAL FIELD

The present disclosure relates generally to the processing of iron-carbon alloys and more specifically to a magnetic field-assisted processing method.

BACKGROUND

The thermal processing of ferrous alloys to achieve good mechanical properties has been widely studied. Most iron-carbon alloys require a rapid quench to reach a metastable martensite phase that can be transformed by heat treatment (tempered) into a desirable microstructure that exhibits good strength and toughness. During the quench, which may occur at a rate of hundreds of degrees per second, the high temperature austenitic phase rapidly cools and is transformed to martensite. Due to the volume expansion (over 4%) that occurs during the phase transformation, tremendous strains may be generated in the iron-carbon alloy during cooling. The surface of the alloy tends to cool more quickly—and thus transform to martensite more quickly—than the interior. When the more ductile interior finally transforms to martensite and expands in volume, the hard martensitic surface is put into tension, and quench cracks may form. As a result, scientists and engineers have tried a myriad of approaches to control the quench process to minimize cracking, such as altering the cooling rate and employing different media for the quench.

Another approach, referred to as austempering, is an alternative to conventional quenching and tempering that avoids forming the metastable martensitic phase in the first place. In austempering, the quench is halted at a temperature above the martensite start temperature, M_(s), and the interior of the alloy is allowed to reach the same temperature as the surface. The temperature is maintained for a time sufficient to transform the entire iron-carbon alloy from austenite to a bainite microstructure, and then the alloy is cooled to room temperature. Consequently, an austempered alloy experiences less distortion and cracking than a conventional quench-and-tempered alloy, and also the energy intensive tempering step is avoided. A downside of austempering, however, is that the transformation from austenite to bainite may take an excessively long time to occur.

BRIEF SUMMARY

Described herein is a magnetic field assisted processing method that may be used to achieve more homogeneous microstructures and improved mechanical properties in ferrous alloys. Using the method, it is possible to produce iron-carbon alloys that include a fine dispersion of iron carbide particles and exhibit a desirable combination of strength and ductility.

The method entails heating an iron-carbon alloy at an austenitizing temperature for a time duration sufficient for the alloy to achieve an austenitic microstructure; cooling the iron-carbon alloy to an intermediate temperature defined by a continuous cooling transformation (CCT) diagram for the iron-carbon alloy at a rate sufficient to avoid phase transformation of the austenitic microstructure, the intermediate temperature being below a bainitic knee of the CCT diagram and above a martensite start temperature; and applying a high field strength magnetic field of at least about 0.2 Tesla to the iron-carbon alloy after reaching the intermediate temperature. The field is applied for a time duration sufficient to transform the austenitic microstructure into a fine dispersion of one or more iron carbide phases in a ferrite matrix in order to produce a magnetically-processed alloy having improved ductility and strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a continuous cooling transformation (CCT) curve for an exemplary iron-carbon alloy and an exemplary cooling path that misses the pearlite/bainite transformation knee of the curve and achieves phase transformation above the martensite start temperature, Ms, before being cooled to ambient temperature;

FIG. 2 shows a series of micrographs of an exemplary ferrous alloy specimen processed according to the method described herein;

FIGS. 3A-3C are micrographs of exemplary ferrous alloy specimens processed with and without a 9T field;

FIGS. 4A-4B show electron backscatter diffraction (EBSD) data and associated grain size distribution analyses;

FIG. 5 shows Charpy V-notch (CVN) versus ultimate tensile strength (UTS) of a series of samples processed with and without a 9 Tesla magnetic field; and

FIG. 6 shows toughness index versus ultimate tensile strength (UTS) of a series of samples processed with and without a 9 Tesla magnetic field.

DETAILED DESCRIPTION

Referring to FIG. 1, which shows a continuous cooling transformation (CCT) diagram for an exemplary iron-carbon alloy, the method entails a first step of heating the iron-carbon alloy at an austenitizing temperature T_(a) for a time duration sufficient for the alloy to achieve an austenitic microstructure, which may be fully or partially austenitic (e.g., the austenitic microstructure may include austenite and a carbide phase in the case of a hypereutectoid alloy). The iron-carbon alloy is then cooled to an intermediate temperature defined by the CCT diagram for the iron-carbon alloy at a rate sufficient to avoid phase transformation of the austenitic microstructure. The intermediate temperature T_(int) is below a bainitic knee of the CCT diagram and above a martensite start temperature of the iron-carbon alloy.

A high field strength magnetic field of at least about 0.2 Tesla is applied to the iron-carbon alloy, preferably after the alloy has been cooled to the intermediate temperature, and the field is applied for a time duration sufficient to transform the austenitic microstructure into a fine dispersion of one or more iron carbide phases in a ferrite matrix. A magnetically-processed iron-carbon alloy having improved ductility and strength may thereby be produced. The fine dispersion of carbide phases is a copious population of second phase (or higher order) carbide particles that can be coherent, semi-coherent, or incoherent with the matrix phase of the alloy. The particles may have an average size in the nanometer to submicron length scale. For example, the particles may be from about 1 nm to about 1 micron in average size. The average size may also lie between about 1 nm and about 500 nm, or between about 1 nm and about 100 nm.

The selection of the intermediate temperature is guided by the CCT curve for the iron-carbon alloy as well as by the Curie temperatures of desired iron-carbon phases, as discussed further below. The method may include holding the iron-carbon alloy at the intermediate temperature during the application of the high field strength magnetic field. The hold time may range from about 1 minute to about 30 minutes. The iron-carbide alloy may be held at the intermediate temperature during the entire time that the magnetic field is applied. Typically, magnetic field is applied for a time duration of between about 1 minute and about 30 minutes.

In some cases, instead of holding the iron-carbon alloy at the intermediate temperature during the application of the high field strength magnetic field, the iron-carbide alloy may be cooled from the intermediate temperature to a lower temperature during the application of the field. The lower temperature may be ambient temperature. The cooling may be carried out by exposing the iron-carbide alloy to a continuous flow of a cooling fluid, which may be an inert gas such as argon or helium, for example, until the lower temperature is reached. The flow of the cooling fluid may be adjusted to achieve a desired cooling rate. Alternatively, the cooling may be achieved by immersing the alloy into an isothermal molten salt or hot oil bath, or fluidized medium bed, or similar approach at the required austempering temperature.

A cooling fluid or one of the alternative cooling methods mentioned above may also be used to cool the iron-carbon alloy to the intermediate temperature from the austenitizing temperature prior to applying the high field strength magnetic field. The iron-carbon alloy may be cooled at a cooling rate that lies between about 1° C./s and 400° C./s, depending on the alloy content and therefore CCT behavior to miss the pearlite/bainite phase transformation knee. The cooling rate during cooling to the intermediate temperature is particularly important; if the rate is not high enough to avoid the knee of the CCT curve, phase transformation of the austenitic microstructure may occur.

The intermediate temperature may be selected to precipitate desired iron and/or other carbide phases in the iron-carbon alloy while avoiding others. If the intermediate temperature is below the Curie temperature of a particular iron carbide phase, then that particular phase may be precipitated during the application of the high field strength magnetic field while other iron carbides having Curie temperatures below the intermediate temperature may be avoided. Table 1 provides a listing of iron carbide phases, including Fe₃C, Fe₇C₃, Fe₂₃C₆, Fe₅C₂, Fe₄C, and others, and their Curie temperatures. Other carbide phases that may be precipitated in the iron-carbon alloy can be represented by including M in the compound name to represent one or more solute elements (i.e., alloy additions such as Mo, V, Cr, Nb, Ti, etc.) that may replace part or all of the iron in the compound. For example, a carbide phase in the iron-carbon alloy may be represented by Fe_(x)M_(1-x)C_(y), where M is selected from among Mo, V, Cr, Nb, and Ti, and x may lie between 0 and 1.

TABLE 1 Compilation of Known Iron Carbide Curie Temperatures Carbide Phase Experimental Curie Designation Temperature, T_(c) (C.) Crystal Structure θFe₃C 228 oP16 χFe₅C₂ 247 mC28 Fe₇C₃ ND hP20 εFe₂C 380 hp ηFe₂C 380 oP6 ΩFe₂₃C₆ ND cF116 Fe₃C ND hP8 Fe₄C ND cP5

The carbide phase(s) that form in the iron-carbon alloy during application of the magnetic field depend on the intermediate temperature and the Curie temperatures of the respective phases, and may be influenced by the magnitude and duration of the applied field. For example, if the intermediate temperature is chosen to lie between about 250° C. and 375° C., then the ferrite matrix may include a dispersion of εFe₂C or ηFe₂C, but θFe₃C and χFe₅C₂ would not be expected to precipitate in the matrix. If the intermediate temperature is chosen to lie between 225° C. and 25° C., then the ferrite matrix may include all of these iron carbide phases. Process parameters may be adjusted to control the size and volume fraction of the precipitated phases in the ferrite matrix since magnetic fields impact nucleation phenomenon significantly.

Magnetic fields increase nucleation in multiple ways. First, the free energy driving force is higher when the precipitating phase is below its Curie compared to the parent phase (that is generally not ferromagnetic) and therefore the nucleation rate would be increased as defined by classical thermodynamic equations. Second, first principles modeling of carbon diffusion in iron is shown to be slowed down and therefore carbon may diffuse a shorter distance in a given amount of time under a high magnetic field, and therefore finer and more copious carbides can be formed under a high magnetic field due to higher free energy driving force. This effect can actually lead to nanoclusters of carbides being formed in an alloy that have never been observed to occur without the presence of a high magnetic field. Also, multiple carbide morphologies can form where normally only one would occur in the absence of a high magnetic field. These effects are shown in FIG. 2, which presents several micrographs¹ of an exemplary iron-carbon alloy after high magnetic field processing according to the present disclosure. The alloy, which was produced by Carpenter Technology Corporation and is further described in Example 1, includes nanoscale carbide clusters and two populations of fine carbides. ¹ Obtained using energy filtered transmission electron microscopy (EF-TEM).

With appropriate control over the type, size and volume fraction of iron carbide phases, it may be possible to produce an iron-carbon alloy having an optimized set of properties, such as simultaneous improvements in both yield strength and toughness, which are normally traded off for optimizing one at the expense of the other.

The high field strength magnetic field may be applied to the iron-carbon alloy only after reaching the intermediate temperature. Alternatively, it may be advantageous to apply the high field strength magnetic field to the iron-carbon alloy during the heating at the austenitizing temperature to increase solubility of alloy additions beyond conventional solubility limits for enhanced solid solution strengthening, as well as after the intermediate temperature is reached. In some cases, the high field strength magnetic field may also be applied to the iron-carbon alloy during the heating to and/or the cooling from the austenitization temperature prior to reaching the intermediate temperature.

The magnetic processing system may be configured to heat the iron-carbon alloy to the austenitizing temperature when the alloy is within the bore of the magnet providing the high field strength magnetic field. Such a magnetic processing system is described for example in U.S. Pat. No. 7,745,765, which issued on Jun. 29, 2010, and is hereby incorporated by reference in its entirety. The austenitizing temperature is generally at least about 25° C. higher than the austenite finish temperature (A_(f)) of the iron-carbon alloy for hypoeutectoid chemistries and between the eutectoid temperature A₁, and A_(cm) (or above A_(cm)), which is the boundary between the austenite and austenite plus carbide phase fields for hypereutectoid chemistries. Alternatively, the heating to and/or at the austenitizing temperature may be carried out in a furnace outside the magnet prior to moving the iron-carbon alloy into the bore of the magnet.

The high field strength magnetic field applied during processing may be at least about 0.2 Tesla, or at least about 1 Tesla. In some cases, the field may be about 10 Tesla or greater, about 20 Tesla or greater, or about 30 Tesla or greater, up to a field of about 40 Tesla. For example, the high field strength magnetic field may lie between about 0.2 Tesla and about 40 Tesla or between about 10 Tesla and about 40 Tesla. Lower fields, for example in the range of from about 0.2 Tesla and to less than 1 Tesla (e.g., 0.9 Tesla), may also be suitable. Ferromagnetic materials such as conventional iron alloys can be magnetized to their saturation limits (e.g., on the order of 1 Tesla) by applying a field as low as about 0.2 Tesla.

Many ferrous alloys may benefit from the method described here. Generally speaking, suitable iron-carbon alloys include steels between about 0.1% carbon and about 2.0% carbon and cast irons containing about 2.0% and about 6.7% carbon (since some cast iron alloys are processed to produce austempered ductile iron). The carbon content may also lie between about 2.0% and about 4.3% carbon. The preceding percentages are given in weight percent. The method is further applicable to carburized and carbonitrided alloys where a chemistry gradient in the surface region is employed to achieve tailored surface properties different from the interior bulk chemistry.

Example 1

The magnetic processing method described above has been applied to low carbon, low alloy steel specimens prepared by Carpenter Technology Corporation (Wyomissing, Pa.). The chemistry of the tested alloy, where the balance is iron, is shown in Table 2.

TABLE 2 Experimental Alloy Composition Element C Mn Si P S Cr Ni Cu V Ti Al N [ppm] O [ppm] wt. % 0.36 0.78 0.94 <.005 <.0005 1.26 3.81 0.52 0.30 <.003 .004 <10 <10

In a series of experiments carried out using this alloy, the following experimental conditions were employed to compare the microstructure and properties when a high magnetic field was applied during the austempering process versus a reference no-field condition.

For the thermomagnetic processing runs, specimens were processed as follows prior to carrying out tensile and Charpy impact tests:

-   -   Austenitized at 885° C. under a 9T field for 30 min;     -   Helium-gas-quenched at 90 psi to 250° C.;     -   Held at 250° C. for 8 hrs under a 9T magnetic field; and     -   Helium-gas-quenched to room temperature.

For the No-Field (NF), 0T processing runs, specimens were processed as follows prior to carrying out tensile and Charpy impact tests:

-   -   Austenitized at 885° C. at 0T/NF for 30 min;     -   Helium-gas-quenched at 90 psi to 250° C.;     -   Held at 250° C. for 8 hrs at 0T/NF; and     -   Helium-gas-quenched to room temperature.

The micrographs shown in FIGS. 3A-3D illustrate the improvement in microstructural homogeneity achieved by processing the Carpenter alloy specimens as described above. FIGS. 3A and 3B are electron micrographs of Carpenter alloy specimens that have undergone a conventional austempering treatment, and FIGS. 3C and 3D are longitudinal and transverse cross section micrographs respectively of the same alloys processed using 9T magnetic fields. The figures show a noticeable distinction in the microstructural response of the alloy by aging with and without a magnetic field imposed. Aging in the presence of the 9T field appears to have avoided the obvious “banding” that results from aging of the same alloy without a magnetic field. The banding may be associated with local chemistry variations.

Referring to FIGS. 4A and 4B, electron backscatter diffraction (EBSD) analyses of the grain size distribution in the longitudinal direction indicate that the Carpenter alloy specimens processed with the 9T field have a finer average grain size than the specimens processed with no field.

In addition, the mechanical properties of the Carpenter alloy specimens processed with a 9T magnetic field show improvements over the conventionally processed alloys. Table 2 below compiles the 0.2% yield strength, % elongation, % reduction in area, and the Charpy V-notch results of the Carpenter alloys processed with the 9T magnetic field and the Carpenter alloys processed conventionally without a magnetic field. As indicated in the table, the properties show increases ranging from 10% to 22%.

TABLE 3 Mechanical Properties of Low Carbon, Low Alloy Steel Specimens Processed With and Without a 9 Tesla Magnetic Field 0.2% Ultimate % Yield Tensile % Reduc- Charpy Hard- strength Strength Elon- tion in V-notch ness (ksi) (ksi) gation Area (ft-lbs) HRC No field 172.69 250.49 15.3 52.08 28.9 49.0 164.05 250.67  13.8*  49.27* 49.0 Average 168.37 250.58 15.3 52.08 28.9 49.0 9 T Mag 189.45 243.65 16.8 58.49 35.3 49.0 Field 188.6 241.42 16.9 59.52 49.0 Average 189.025 242.535  16.85 59.01 35.3 49.0 Property 12.30% −3.20% 10% 13%  22.10% 0.0 Change with H-field *Note: Specimen broke very near line mark. % Elongation and % R.A. may not be reliable.

FIG. 5 shows Charpy V-notch (CVN) versus ultimate tensile strength (UTS) of a series of samples processed with and without a 9 Tesla magnetic field, and FIG. 6 shows toughness index versus ultimate tensile strength (UTS) of the same series of samples processed with and without a 9 Tesla magnetic field. These figures demonstrate that thermomagnetic processing improves the combination of strength and impact energy/toughness over this alloy's performance without a high field applied during the identical no-field processing conditions. Similarly, FIG. 6 highlights that this magnetically processed alloy provides the same toughness as a commercial alloy, Hy-Tuf, known for its toughness performance while delivering higher strength capability. FIG. 5 shows the outstanding performance of this magnetically processed lower cost, low alloy content alloy over much more expensive, high alloy content materials such as the Maraging 250 specialty alloy steel.

Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible without departing from the present invention. The spirit and scope of the appended claims should not be limited, therefore, to the description of the preferred embodiments included here. All embodiments that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein. Furthermore, the advantages described above are not necessarily the only advantages of the invention, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the invention. 

1. A method of magnetically processing an iron-carbon alloy, the method comprising: heating an iron-carbon alloy at an austenitizing temperature for a time duration sufficient for the alloy to achieve an austenitic microstructure; cooling the iron-carbon alloy to an intermediate temperature defined by a continuous cooling transformation (CCT) diagram for the iron-carbon alloy at a rate sufficient to avoid phase transformation of the austenitic microstructure, the intermediate temperature being below a bainitic knee of the CCT diagram and above a martensite start temperature; and applying a high field strength magnetic field of at least about 0.2 Tesla to the iron-carbon alloy after reaching the intermediate temperature, the field being applied for a time duration sufficient to transform the austenitic microstructure into a fine dispersion of one or more iron carbide phases in a ferrite matrix to produce a magnetically-processed alloy having improved ductility and strength.
 2. The method of claim 1, wherein the intermediate temperature is below a Curie temperature of the one or more iron carbide phases in the ferrite matrix.
 3. The method of claim 2, wherein the intermediate temperature is above a Curie temperature of one or more iron carbide phases not present in the ferrite matrix.
 4. The method of claim 1, wherein the one or more iron carbide phases in the ferrite matrix are selected from the group consisting of Fe₃C, Fe₇C₃, Fe₂C, Fe₂₃C₆, Fe₄C, and Fe₅C₂.
 5. The method of claim 1, wherein at least one of the iron carbide phases further comprises a metal M selected from the group consisting of Mo, V, Cr, Nb, and Ti.
 6. The method of claim 1, further comprising at least one additional metal carbide phase in the ferrite matrix, wherein the metal carbide phase includes a metal M selected from the group consisting of Mo, V, Cr, Nb and Ti.
 7. The method of claim 1, wherein the intermediate temperature lies between about 150° C. and 550° C.
 8. The method of claim 1, further comprising holding the iron-carbon alloy at the intermediate temperature during the application of the high field strength magnetic field.
 9. The method of claim 1, further comprising cooling the iron-carbon alloy from the intermediate temperature to a lower temperature during the application of the high field strength magnetic field.
 10. The method of claim 9, wherein the lower temperature is ambient temperature.
 11. The method of claim 1, wherein the time duration of the application of the magnetic field after reaching the intermediate temperature is between about 1 minute and 30 minutes.
 12. The method of claim 1, wherein the rate of cooling of the iron-carbon alloy to the intermediate temperature is between about 1° C./s and 400° C./s.
 13. The method of claim 1, further comprising applying the high field strength magnetic field to the iron-carbon alloy during the cooling of the alloy to the intermediate temperature.
 14. The method of claim 1, wherein the high field strength magnetic field is between about 0.2 Tesla and about 40 Tesla.
 15. The method of claim 14, wherein the high field strength magnetic field is in the range of from about 0.2 Tesla and to less than 1 Tesla.
 16. The method of claim 1, wherein the heating at the austenitization temperature occurs in a furnace outside a high field strength magnet, and further comprising moving the iron-carbon alloy from the furnace into a bore of the magnet during the cooling to the intermediate temperature.
 17. The method of claim 1, further comprising applying the high field strength magnetic field to the iron-carbon alloy during the heating at the austenitizing temperature.
 18. The method of claim 1, wherein the high field strength magnetic field is applied to the iron-carbon alloy only after the alloy has reached the intermediate temperature.
 19. The method of claim 1, wherein the austenitizing temperature is at least about 25° C. higher than an austenite finish temperature of the iron-carbon alloy.
 20. The method of claim 1, wherein the iron-carbon alloy includes between about 0.1% carbon and about 6.7% carbon. 